Residual―heat Utilization and Optimal Reformation in PVC production

Abstract： The paper takes the actual problems of target enterprises in PVC production as an example. The paper transforms residual-heat waste status in production and optimizes the key equipment， cooling water system of internal cooled of polymerization reactor， which not only reduces energy waster and reaction time of single reactor， but also improves production efficiency. So it saves energy for target enterprises and makes considerable economic benefit.

Key words： PVC， residual-heat utilization， polymerization reactor

1 Introduction

PVC is an important organic synthesis material in China. It is widely applied to the fields of industry， construction， agriculture， electric power and package. For the product category of PVC， it belongs to three synthetic materials， synthetic resin， synthetic fiber and synthetic resin in synthetic rubber. The production and sales of it is only after polyethylene in the world 1. However， China is the country producing and consuming the most PVC in the world， so the production technology of PVC has great influence on China. Polymerization reactor is one of the most important equipment in a set of production equipment of PVC. The technology of polymerization reactor has direct influence on the quality and production of PVC. Therefore， the quality of polymerization reactor has close relationship with economic benefit of enterprises.

As a large energy consumer， low energy utilization is a problem which can’t be ignored in China. The average residual-heat utilization rate in each industry has reached to 7.3%， but the recovery rate of residual heat resources is only 34.9%， which indicates that residual heat resources have great potentials 2. For residual-heat waste status in production of target PVC enterprises， the paper optimizes the production process， which improves the economic benefit and energy utilization rate of target enterprises.

2 Situation of PVC Industries in China

At present， PVC industries in China have been in the industrial environment of global completion， Asian integrated completion and domestic low-degree completion. With the expansion and emergency of new chlorine-alkali enterprises， the global chlorine-alkali enterprises have been in the era of supply exceeding demand.

For example， the chlorine-alkali enterprises continued the growth trend in China last year. The productivity of caustic soda increased by 3.23 million t/a and reached to 37.36 million， which increased by 9.5% compared with that in last year. The annual yield was 26.986 million t， which increased by 3.8%. In last year， the production efficiency of PVC increased by 1. 72 million t/a， and reached to 23.41 million， which increased by 8.2% compared with that in the last year. The annual yield was 13.178 million t， which increased by 0.5%3. The chlorine-alkali enterprises faces complicated and strict survival environment in China. Influenced by sustainable growth of productivity， weakness of downstream demand， increase of comprehensive cost and poor export， chlorine-alkali enterprises face great survival pressure， and the overall efficiency of the industry decreases. Many enterprises take the production mode of using alkali to supplement chlorine， but it has no good effect. With the decrease of caustic soda market in China， the price of PVC products has no evident increase， which makes the survival status of chlorine-alkali enterprises deteriorated. The issue of Enhancing Calcium Carbide Manufacture PVC and Pollution Control of Mercury limits the application of high-mercury catalyst， and it demands the PVC enterprises must use low- mercury catalyst 4. It makes the enterprises increase the relevant manpower and financial investment， which increases the burden of enterprises. Therefore， it is has great significance for enterprises to improve productivity and improve energy utilization efficiency.

3 System Overview

1.Introduction of target enterprises

The selected target enterprise is a military enterprise established in 1996. The enterprise was transformed into company system from factory system in 1996. In 2010， the enterprise became a state-owned enterprise， and the annual sales revenue was more than 4 billion yuan. It is a large state-owned industrial enterprise which is predominated by chlorina alkali chemical industry and is equipped with thermoelectricity， cement， PVC deep process and photovoltaic industry. The annual scale of the leading products including ionic membrane caustic soda and PVC resin is 540000 tons and 500000 tons. There is an owned cogeneration power plant with 152000 KW of installed capacity and a carbide slag producing cement enterprise with annual output of 1 million tons. And it has complete production structure.

2 Factory before transformation

2.1 Heat-thermal waster status in drying section

The target enterprise has the matched power plant. In order to make PVC production easy， the power plant transports some steam with high temperature and high pressure to dry processes of PVC production for application. In PVC production process of target enterprise， the drying processes use vortex drying bed. The production principle of vortex drying bed is using high temperature and hot air to make PVC resin dry in cyclone drying tower. And the heat air is from air heater. The vortex drying bed has the characteristics of easy process， great processing capacity， few of dead angles and easy to be cleaned， which has evident function to improve productivity of equipment and product quality. But the disadvantage is that steam consumption is great， which is very evident. The best consumption level of vortex drying bed is 0.5t/t・PVC. But the difference of technical level， operational norms and regional environment makes the consumption of steam different. In the production process in winter in the northern area， the steam consumption is greater.

According to the actual situation of target enterprises， after the steam with high temperature and high pressure makes heat exchange with cold air in air heater， the cold air becomes high-temperature air for drying process. After the high-temperature steam receives heat exchange， some become condensed water， and most steams become low-pressure steam as tail gas to be discharged into air， which means that most heat is wasted as steam condensate and low-pressure steam， which causes severe loss. Therefore， it is necessary to effectively make use of the lost energy.

2.2 Efficiency of polymerization workshop is not good

The project of 200000 tons of PVC of the target enterprise in the paper was established in August 2006 and was put into production. Polymerization reactor is the key large-scale equipment in polymerization workshop. The enterprise uses 70m?polymerization reactor. And the structure chart is as follows.

1.transmission；2impeller；3 agitating shaft；4 reactor；5 half-pipe jacket；6 safety device；7manhole；8 spray device；9 inner cooling tube；10 baiting valve

Figure： 70m?agitated reactor 5

The heat-transfer capability of polymerization reactor is one of the key factors restricting production capacity 6. The polymerization reactor of target factory uses two-way cooling device. One way is cooling in half-pipe jacket outside of polymerization reactor. And the other way is cooling in four inner cooling pipes inside polymerization reactor. Two-way cooling water uses circulating water for cooling， but circulating water is restricted by natural conditions. For target enterprises， the average reaction time of PVC5 polymerization reactor is from 340min to 350min， and the temperature of circulation water is from 24℃ to 25℃. Under the condition that jacket of polymerization reactor and regulating valve of inner cooling pipe are nearly open， the polymerization time long， and the temperature and pressure of polymerization reactor in reaction process rises， which not only needs to reduce the temperature in polymerization reactor by fetching water， but also needs to reduce the usage of initiator. And the reaction time delays， which is not good for production. In June， July， August and September， the external temperature is high， and the temperature of circulation water rises to 28℃. Too high temperature of circulation water makes the time of PVC monomer polymerization in the operation process longer than that in winter， which seriously affects production efficiency. Therefore， reducing reaction temperature of polymerization reactor and reducing polymerization time has great significance to improve production efficiency in polymerization stage.

3. Transformation method

In order to save energy， reduce emission and improve the benefit of enterprises， according to the characteristics of target enterprise， we make appropriate transformation on enterprises. Firstly， in order to use the waste heat expelled in drying process of PVC production， we add a steam double-effect lithium bromide absorption cooling-water machine in drying process， and use the heat provided by waste heat to produce cooling water with the temperature of 7℃. And the produced cooling water is sent to polymerization workshop in PVC production for reducing the reaction temperature of polymerization reactor.

3.1 Utilization and transformation of waste heat in drying section

The data provided by target enterprises indicates that the temperature of the waste gas exhausted in drying section is 160℃～180℃， the pressure is 0.18MPa and the quantity of steam is 710kg/h.

From the data provided in the table， we see that the quantity of steam which can be used is about 710kg/h. According to refrigeration air conditioner equipment manual and the data provided by the relevant manufacturers 7， RCW-032-type steam double-effect lithium bromide absorption cooling-water machine is selected （parameters are shown in the following table）. The normal steam demand with normal operation is 704kg/h， and the quantity of steam provided in drying section of target enterprise can meet the demand. The refrigerating capacity of the unit is 563kW， the working pressure is 0.4MPa， and cooling water of 7℃ is provided with 163m?/h.

3.2 Transformation of cooling water in polymerization workshop

In order to improve heat-transfer capability and production efficiency of polymerization reactor， the paper makes improvement on the original working conditions， as follows. An inlet is added to the original circulating water pipe. And the water with the temperature of 7℃ which is produced in steam double-effect lithium bromide absorption cooling-water machine is led into inner cooling pipe of polymerization reactor for cooling water in inner cooling pipe. The cooling water with the temperature of 7℃ replaces industrial circulation water to be led into inner cooling pipe. As the temperature of cooling water is low， the heat transfer temperature difference is great， the heat transferring effect is better， which reduces the reaction temperature of polymerization reactor.

As shown in the following figure， before transforming cooling water system in polymerization section， the industrial circulating water required by reactor is provided by the circulating water station， and the original working condition has no branch A， B and C. The circulating water is send to reactor by D pipeline of water pump， which cools polymerization reaction. After transformation， branch C providing cooling water with the temperature of 7℃is added， which reduces the reaction time of polymerization reactor.

1 Steam double-effect lithium bromide absorption cooling-water machine 2Circulation water station 3 circulating water pump 4 Reactor

In the figure， branch A is from drying section and provides high-temperature steam. Branch C provides the cooling water with the temperature of 7℃ which is provided by lithium bromide chiller， and it is used to cool inner cooling tube of polymerization reactor. Branch D is the industrial circulating water provided by industrial circulating water station， and is used to cool half-pipe jacket outside of polymerization reactor. F is the exit of reactor. In order to be safe， while transforming inner cooling tube， the cooling water with the temperature of 7℃ and circulating water is reserved mutually， which not only increases the flexibility of operation， but also avoids accidents.

Table： Comparison on indicators of 5℃ cooling water and circulating water

4 Design Scheme Accounting

1.Design foundation

According to the actual production condition， the maximum flux of inner cooling tube is qmax1=280m?/h， and that of jacket of polymerization reactor is qmax2=480m3/h. The inner cooling tube of polymerization reactor uses industrial circulating water. The polymerization reaction time of PVC8 type is 360min～370min in summer， and that of PVC5 type is 340min～350min， and that of PVC3 type is 310min～320min.

In order to effectively improve polymerization productivity in summer， the industrial circulating water used for inner cooling tube of polymerization reactor is replaced by 7℃ cooling water. Reducing the backwater temperature can improve heat-transfer efficiency and reduce polymerization time， which improves productivity.

2.Design scheme

2.1 Calculation conditions

The polymerization batch charging is 25t/reactor， PVC polymerization reaction heat is 22.9kcal/mol， the reaction conversion rate is 80%， the original reaction time is 5.5 h， and the reaction temperature is 56.5℃.

2.2 Heat balance and material balance

2.2.1 Maximum thermal load of polymerization Qmax

From the drawings of polymerization reactor， we can know that the heat exchange area of inner cooling tube is F1=19m2， and the heat exchange area of jacket is F2=80m2. In practical production process， the jacket and inner cooling tube nearly receives heat transfer by fetching water. And the heat distribution problem can be distributed according to heat transfer area， Q single inner cooling：Q single jacket=19：80.

Reaction exotherm of single reactor is Q single reactor：=25×1000×22.9×80%×1000/62.5=733×104kcal， Q single inner cooling tube =Q single reactor×19/99=141×104kcal， and Q single jacket=Q single reactor×80/99=592×104kcal

2.2.2 Reducing polymerization reaction time

If 7℃ cooling water is used for heat transfer， and the water intake still uses the original industrial water pipeline， the maximum flux is 280m3/h. After using 7℃ cooling water， heat exchange rate accelerates， and the reaction time reduces.

In summer， the circulating water ingress temperature is 28℃， and the outlet water temperature is 30℃， and the reaction time is 5.5 h. According to the given conditions， F1=19m2， t2=56.5-（30+28）/2=27.5℃， Q single inner cooling tube =Q single reactor×19/99=141×104kcal=K×F1×t2×T， we can solve heat transfer coefficient K=490kcal/（m2.h.℃）.

When the water in inner cooling pipe is replaced by 7℃ cooling water， the reaction is divided into two parts. The water inlet temperature of cooling water in summer is 7℃， and the water outlet temperature is 9℃. The first part of time is T1：F1=19m2， t1=56.5-（7+9）/2=48.5℃， Q single inner cooling tube=Q single reactor×19/99=141×104kcal=K×F1×t1×T1， which can get T1=3.12h. And the second part of time is T2：Q jacket heat=592×104kcal×（5.5-3.12）/5.5=256×104kcal， Q jacket heat==（K×F1×t1+K×F2×t2）×T2， which can get T2=1.67h. The inner cooling heat of the second part of time is QT2=75.4×104kcal， T总=T1+T2=4.8h. And the reaction time can be reduced by 0.7h8.

Theoretically， the maximum shortening reaction time is 0.7h. As the heat is not released averagely in actual reaction process， and the flux adjusts with the temperature change of reactor， actual reaction time is longer than theoretical calculation， and the shortening reaction time is 0.5h.

The temperature difference between fetch water and backwater of 7℃ cooling water is t=2℃， ρ×q average flux×Cpm×t=K×F1×t1， q average flux =225m?/h（ if the temperature difference between fetch water and backwater of 7℃ cooling water is increased， the average flux of 7℃ water reduces）.

2.2.3 Accounting required cold capacity

The cooling capacity of ice maker required by single reactor is （Q single inner cooling+QT2）/T total=45.1×104kcal/h， and there are five reactors in polymerization A section and B section. In real production process， not all reactors react. The reaction of polymerization A section and B section can be considered separately.

Situation 1： There are six reactors reacting in polymerization A section and B section. The required quantity of 7℃ cooling water is QA=225m3/h×3=675m3/h， the required cooling capacity of ice maker is WA=45.5×104kcal/h×3=136.5×104kcal/h. Combing polymerization A section and B section： the required quantity of 7℃ cooling water is Q total=675m3/h×2=1350m3/h， and the required cooling capacity of ice maker is W total=136.5×104kcal/h×2=273×104kcal/h.

Situation 2： If polymerization A section and B section are combined， there are 7 reactors reacting. Polymerization A section： QA=225m3/h×3.5=787.5m3/h. The required quantity of 7℃ cooling water is Q total=m3/h×7=1575m3/h， and the required cooling capacity of ice maker is W total=45.5×104kcal/h×7=318.5×104kcal/h.

Situation 3： There are 8 reactors reacting for combining polymerization A section and B section. The required quantity of 7℃ cooling water is Q total=900m3/h×2=1800m3/h， and the required cooling capacity of ice maker is W total=45.5×104kcal/h×8=364×104kcal/h.

3Scheme selection

Combined with the actual conditions， only polymerization B section is transformed. The consumption of 7℃ cooling water is close to situation 2. Polymerization B accesses to the header pipe of DN350mm from the header pipe of 7℃ cooling water， and is connected with5℃ cooling water， which still uses water inlet of inner cooling tube. The maximum flux has no change. A valve is added to industrial circulating water pipe of the original accessed inner cooling pipe， which makes it switch with 5℃ cooling water. The flow rate of header pipe of DN350mm is 3m/s. Q=3600s×（0.350/2）2×π×3m/s=1039m3/h， the backwater temperature of 7℃ cooling water is lower than that of industrial circulation water， which meets the production requirements.

DN350mm is added to 7℃ backwater pipe of DN400mm in polymerization B section， and two booster pumps are added. One pump is used， and the other is spared. A converter is installed on pump. And the stable water pressure is 4.5kg.

5 Cost Accounting and Economic Analysis

1Cost accounting

The steam double-effect lithium bromide absorption cooling-water unit is about 1.5 million yuan.

The installation and transformation of pipelines is about 80000 yuan.

Two pipeline pumps are 120000 yuan.

Two frequency converters and control cubicle are 40000 yuan.

It means that the total investment of equipment is 1.74 million yuan.

2. Economic analysis

After polymeric formula adjustment makes reaction time reduce， the extra 24/5-24/5.5=0.43 reactors are produced by every reactor every day. The time using 7℃ cooling water is in the second half of June， and the first half of July， August and September. And the time is 100 days. The total output of PVC is 0.436×25×80%×100×10=8718t/a.

According to the present market prices of PVC， each ton is 6300 yuan. Under the condition of deducting material expense， labor cost， electric charge， water fee， environment pollution charge， equipment maintenance and depreciation cost， each PVC profits 130 yuan， and the transformed scheme can increase the profit of 130×8718=1133340 yuan for enterprises.

The transformation cost in early state is 1.74 million yuan. So after 18 months， the enterprises can regain the transformation cost and realize sustainable profit.

6 Conclusion

As one of the most important equipment of PVC production， the production state of polymerization reactor is very important. And improving efficiency of polymerization reactor is the manifestation of corporate strength. It is an important manifestation of realizing energy-efficient production for enterprises to use waste heat， conserve energy and reduce emission. Using waste heat in drying section of target enterprises and transforming inner cooling tube of polymerization reactor in polymerization section fully indicates that it is effective to use waste heat to produce cooling water for reducing reaction temperature of polymerization reactor. Reducing polymerization time not only improves production efficiency of enterprises， but also makes objective productivity effect.

From the transformed theoretical calculation， we can see that using waste heat not only reduces energy wastage， but also reduces energy input of enterprise. Transformation on inner cooling pipes not only improves conversion rate of single reactor， but also reduces production pressure of equipment and improves production efficiency of polymerization reactor. Transformation on waste heat utilization in drying section and transformation on cold water system of inner cooling tube of polymerization reactor has the characteristics of using less material expense， little construction difficulty， and short construction period， which makes high economic profit. So it is suitable for technological transformation of production equipment of enterprises.

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